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Re: NTP, possible solutions, and best implementation

  • From: David G. Andersen
  • Date: Thu Oct 02 14:27:09 2003

Really good back-end clocks, particularly when you're
implementing a redundant system with a cesium clock or
two, are a lot more expensive than the front-end servers
that clients need to see.

Do you have particular technical reasons you want to put the
"actual device" on the net?  Most of the time, the "actual 
device" in these cases is merely a linux box or something
similar wrapped around a stratum-zero time source.

A much easier way to do this, which gives you control over
what faces the network, is to run your own front-ends of
stratum 1 servers that all talk to the same network of
back-end time sources:

     FE-1      FE-2       FE-3
      |         |          |
       \--Time Distribution/
        \       |         /
        GPS    Cesium   Clock3

Put cards in the front ends that let them receive PPS or
IRIG, and then let them all get a time signal from the
back end clocks.  The clocks on the FEs will be good to
a handfull of microseconds.  since the FEs will be
synched directly to a stratum zero time source (the PPS
signal from the GPS and cesium clocks, for instance),
they'll be running at stratum 1.

As a nice example of how you can structure this, see the
udel NTP page.  The added expense here will be
purchasing an IRIG board for the front-ends, or you can
decode it via the sound input, but if you go with a real
irig board, you'll get a high-quality onboard oscillator
that'll do a much better job of running free if it loses
the backends.

This way, you can have as many front ends as you need for
(redundancy, scalability, ...) and they'll run a real OS
that you control.  No need to put an application level
forwarder in the middle, since they _are_ your application
level gateways to the PPS/IRIG signal.

btw, comp.protocols.time.ntp is a good source of information
about this kind of thing.. probably better than nanog.

  -Dave

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